Color printing requires registration of the printing plates on the cylinders of the print engine to achieve acceptable print quality. For this purpose many known in the art devices are available on the print cylinders, including, for example, registration pins and clamps. Since the early nineties certain types of printing plates can be prepared on Computer to Plate (CTP) imagesetters such as, for example, the Trendsetter by Creo Inc. of Vancouver Canada or the Lotem series by Creo IL Ltd. of Herzelia Israel. The imaging of a set of printing plates (for example, four CMYK separation plates) requires close registration as well, in purpose to achieve the quality of the printed image represented by that set of plates. This is achieved in imaging of Offset printing by the use of, for example, registration pins on the imaging cylinder of the CTP device and pre-punched registration holes in the separation plates. The use of imaging cylinder registration pins and punched holes in the imaged plates conform to the method of registration used on the corresponding printing press. The same imaging cylinder registration pins and punched holes can be further utilized for the imaging related “spiral compensation” as described in U.S. Pat. No. 6,016,752 assigned to Creo IL Ltd. of Herzelia, Israel.
Plates for flexography, used, for example in decorative printing, having a polyester base, are imaged on known in the art CTP thermal imagesetters, such as the ThermoFlex by Creo. The imaging cylinder in such CTP imagesetters got short registration pins, as will be further explained, and circumferential vacuum slots to enable firm attachment of the plate during operation.
CTP Letterpress (raised type) printing plates are available in both polyester and ferromagnetic-metal base. The polyester base plates can be imaged on the Creo Thermoflex imagesetter, but this is impractical for other types of Letterpress plates having a ferromagnetic-metal back (or base), for example, BASF of Germany model WS73D used, for example, in printing on metal cans. Such plates are much heavier and require better attachment to the cylinder means as to prevent the centrifugal forces to tear the plate from the fast rotating cylinder and cause damage to the imagesetter. While mechanical plate clamps, as known in the art, can not be used for reasons to be further explained, and vacuum holding is not safe, a good solution is to use a “magnetic imaging cylinder” (or “magnetic drum”) as part of the system. The known in the art “magnetic cylinder”, used in certain printing presses, enables effective attachment and holding of ferromagnetic metal-based plates during the imaging operation. FIG. 1 depicts schematically a known in the art “magnetic drum” 10 as used, for example, in Letterpress printing devices. The cylinder 11 is made of non-magnetic material, in which rows of magnets 16 are embedded. These magnets 16 keep the ferromagnetic-metal backed plate firmly attached to the rotating cylinder during normal operation. To enable correct registration of the plate on the drum registration pins 14 are available as known in the art. Additional strong bar magnets 12 are optionally embedded in the non-magnetic cylinder, in the same row and next to the registration pins 14, to ensure firm attachment of the plate's leading and trailing edges.
A typical Letterpress printing plate 20, of the polyester or ferromagnetic metal back types, is shown schematically in FIG. 2, wherein the relief thermal imaging polymer layer 22 got a back (or base) 24 of polyester or ferromagnetic metal. Typically the Relief 22 depth Dp is thicker than the ferromagnetic-metal base 24, for example:
A BASF (Germany) Letterpress steel backed plate, type WS73D got a Relief depth of Dp=0.65 mm and a base of Ds=0.27 mm.
The thermal imaging process, requiring a good focus of the imaging energy on the Relief part, dictates the use of an automatic focus device as known in the art. During the automatic focusing operation the imaging head (not shown in FIG. 2) must be able to approach the surface 26 of the relief layer 22, which dictates a severe limitation that no mechanical obstacle, including registration pins, should protrude more than (for a specific imagesetter, as an example) about 0.5 mm above the plate surface 26 of the least thick Letterpress plate in use. Having short registration pins 14 on a “magnetic drum” 10 combined with the strong magnetic forces of magnets 16 in the area of the registration pins create a severe limitation on the maneuverability of the plate's leading edge during a manual attempt to register the plate on the imaging cylinder, as the magnets 14 “snatch” and lock the ferromagnetic metal base of the plate as soon as the gap between magnets and metal base narrows sufficiently. A similar problem occurs in printing presses, having a “magnetic printing cylinder”. According to one prior art method of overcoming the above mentioned difficulties, the entire “magnetic printing cylinder” is removed from the press to an external “registration table” where the plate registration slots are registered optically to the printing cylinder pins, in a position that minimizes the interference of magnetic forces. Following the registration of the plate's leading edge the “magnetic printing cylinder” is rotated to wrap around the entire plate. The ready for print cylinder is subsequently replaced in the press. This prior-art registration procedure is cumbersome and costly.
Another related requirement concerns CTP devices only, as these machines are designed for various sizes of media. The registration pins on the imaging cylinder must be replaceable and/or movable to conform to various standards of registration slots available in the art.
The invented registration method described herein solves the above mentioned constraints and offers an easy to operate, accurate and versatile method of mounting and registering Letterpress plates on “magnetic drum” imagesetters.